Three dimensional (3D) time-of-flight (TOF) cameras are capable of getting depth information for all pixels in a two dimensional array in real-time.
The distance resolution of TOF cameras is, among other factors, limited by the number of electrons which can be stored in the pixel. 2D-cameras are designed such that their pixels can store enough charge carriers to reach the desired signal-to-noise ratio. 3D-TOF imagers also need to store enough electrons that are generated by the modulated illumination in order to achieve a minimum distance measurement resolution. In addition, the imagers have to cope with electrons generated by any background light.
TOF pixels, as presented in [MOE05A], [KAW06A], [GOK04A], [NIE05A], use a diffusion capacitance for electron storage, the so-called sense node. The maximum possible number of electrons in the sense node depends on the size of the sense node and the capacitance per unit area. This means, for a large optical fill factor and high sensitivity of the pixel, the capacitance per unit area must be as high as possible. However, the capacitance for a diffusion node is determined by the technology parameters and cannot be increased further by the chip designer.
The demodulation of modulated light signals accomplished already at pixel level requires, in all approaches known today, the switching of a photo-generated current. It is possible to handle both electron as well as hole currents, but the common methods make use of the photo-generated electrons due to their higher mobility in the semiconductor material. Some pixel architectures do the necessary signal processing based on the photo-current and other architectures work in the charge domain directly.
Common to all pixels is the necessary transfer of charges through the photo-sensitive detection region to a subsequent storage area or to a subsequent processing unit. In the case of charge domain-based pixel architectures, the photo-charge is generally transferred to a storage node. In order to demodulate an optical signal, the pixel has to have implemented at least two integration nodes that are accumulating the photo-generated charges during certain time intervals. Another minimum requirement would be the implementation of at least one integration node and having at least one other node for dumping charge carriers as well.
Different pixel concepts have been realized in the last few decades. Spirig [SPI95B] and later Schwarte [SCH98A] introduced a demodulation pixel, which transfers the photo-generated charge below a certain number of adjacent poly-silicon gates to discrete accumulation areas. Spirig [SPI99A] disclosed a CCD lock-in concept that allows the in-pixel sampling of the impinging light signal with theoretically an arbitrary number of samples. First cameras based on this lock-in pixel concept have been demonstrated by Lange [LAN99A] and Oggier [Ogg03A]. Another similar pixel concept has been demonstrated by Kawahito [KAW06A], [KAW07A], where a thick field-oxide layer is used to smear the potential distribution below the demodulation gates.
New concepts for pixels have been explored in the last years that involve accelerating the in-pixel transport of the charges by exploiting lateral electric drift fields. Seitz [SEI02A] invented the first drift field demodulation device that is based on a very high-resistivity polysilicon gate electrode. It even allows very easy design of pixels that implement an arbitrary number of samples. The concept was proven by Buettgen [BUE05B], who disclosed later another concept of demodulation pixels. The static drift field pixel [BUE04B] [BUE05A], in contrast to the architectures mentioned before, clearly separates the detection and the demodulation regions within the pixel. Another pixel concept was proven by Nieuwenhoven [NIE05A], in which the lateral electric drift field is generated by the current of majority carriers within the semiconductor substrate. Minority carriers are generated by the photons and transported to the particular side of the pixel just depending on the applied drift field.
All of the above concepts must be able to store a large number of charge carriers within the pixel itself in order to reach a high signal quality, and therefore all of them depend on an efficient storage structure. This means the more electrons can be stored in the pixel, the better the distance resolution, or in the case of background (BG) light-dominated scenes, the better the BG suppression.
The trend of 3D imaging pixels is going in the direction of integrating n>=3 storage nodes per pixel. This, however, claims not only more space for the n-storage-gates, but also requires an integration of m<=n source followers, m select transistors, and m reset transistors in every pixel. The problem here is that the pixel's optical fill factor gets smaller when integrating a larger number of storage nodes and output paths, which, in turn, decreases significantly the sensitivity of the pixel.
In most state-of-the-art pixels the charge is stored in diffusion capacitances in the so-called sense nodes. However, the diffusion capacitance is rather small compared to other capacitance types and claims more space for a given amount of electrons. In 3D TOF pixels, the area spent for storing electrons is one of the most area-consuming parts in the pixel. In order to increase the pixel performance, especially in multi-tap pixels, new storage structures must be found.
Furthermore, diffusion in the sense node is affected by interface defects, which increase the noise level while storing the charge therein. In contrast, gate capacitances allow a more efficient, noise-reduced storage, but they have the disadvantage that the charge under the gate cannot be tapped.